How to set backround image in Blender 2.8
You need to add a Texture Coordinate node and connect it to your Image Texture node, otherwise blender won't be able to map the texture properly. You can then move, rotate and scale the texture using a Mapping node.

For comparison, without Texture Coordinate node:



And with:



EDIT: After seeing your screenshot, I see the issue: you are editing a MATERIAL node, when you should be editing the WORLD node. Simply select "World" from the dropdown list on the upper left corner: